A personal digital notepad (PDN) is a digitizer-and-pen device that allows a writer to record handwritten information on a standard paper notepad, and simultaneously record an electronic carbon copy of the writing in electronic form. The CrossPad marketed by the A.T. Cross Company is an example of such a device as is the IBM ThinkScribe. For such a device to be useful, the electronic ink on the electronic page must mirror the physical ink on the paper page. In order to maintain this accurate correspondence between the physical page and the electronic copy, the writer is required to “turn” the electronic page when changing to a new or previous paper page by pressing the corresponding page-forward or page-backward button on the PDN. These buttons effect synchrony between the physical and electronic page by recording these events in the data stream. Asynchrony between the paper and electronic pages occurs when a writer forgets to press the appropriate button on the device or accidentally presses the button too many times. Subsequent writing is then electronically recorded on the wrong electronic page, and the new electronic ink is recorded on top of the page's original electronic ink. This problem may be compounded since the user may flip forward or backward by several pages at a time and may do so several times within a single document. Later, when the resultant electronic page is viewed, the merged original and overwritten electronic ink can be confusing and may be difficult to read and correct.
Ink data collected by a PDN is recorded as a sequence of triplets (X, Y, T) corresponding to the horizontal position, the vertical position, and the timestamp, respectively, of the pen tip on the page. These triplets may be sent to a personal computer (PC) in real-time or stored on the PDN and uploaded at the writer's convenience. Thus, manipulations of this data may be performed in real-time, possibly with writer-intervention, or they may be performed at any later time as a post-processing step. In addition to the triplets, the PDN may also include other information, called events, in its data stream, such as the location of the pen (on/off the paper) and whether the writer has activated any special purpose buttons on the device. For the purposes of the present invention, we need only concern ourselves with the triplets, page forward/backward events and document description information.